Offshore structures within the meaning of the present invention are, for example, transformer substations, wind turbines, or drilling or production platforms.
For particular types of foundation for offshore structures, in particular for offshore wind turbines, a piled foundation in the seabed is required. Commonly used foundation types are, for example, so-called monopiles, jackets, tripods or tripiles. Thus, jackets, for example, are framework structures of steel tubes, which have a triangular or quadrangular base and the upper end of which, following installation, projects out of the sea. A conventional wind turbine, a transformer substation or a drilling platform, for example, can be erected on a jacket.
In the case of a jacket foundation, in the operation of placing the piles the latter have to be positioned with a predefined spacing on the seabed, in the so-called “pre-piling” operation, this being effected by means of corresponding templates. This presupposes that the nature of the ground is such that all the piles to be placed can be driven as far as a predefined load-bearing ground horizon of the seabed. There are no possibilities for variation with respect to the spacing or siting or the placing of the piles.
In the case of a jacket foundation, a steel-lattice structure/steel-tube structure is then placed on the steel piles, the structure receiving a so-called “transition piece” above surface of the sea. The transition piece then receives the actual structure, for example in the form of a transformer substation.
To that extent, monopiles are preferably used in the installing of the foundation of wind turbines in water depths of up to 35 m, because this type of foundation is less elaborate. Usually, there is at least one transformer substation assigned to the wind turbine, positioned in immediate proximity to the wind turbine. In the transformer substation, the voltage generated by the wind turbine can be transformed, for example, from 33 kV to 155 kV and rectified, in order to be transmitted with low loss over large distances by means of high-voltage direct-current (HVDC) transmission systems.
It is also desirable to use monopiles for the foundation of transformer substations because, as already described above, they are very much easier to install than, for example, a jacket. Owing to the large extent and, in particular, owing to the large mass, of a transformer substation, of up to more than 1000 tons, the latter constitutes a large oscillatory mass in the case of installing the foundation on a monopile. The action of wind on the transformer substation and the action of wind and waves on the monopile result in excitation of oscillations of the entire structure, which oscillations on the one hand negatively affect the structural integrity of the monopile and of the transformer substation and, in addition, may cause nausea for personnel present on the transformer substation.
EP 2 743 170 A1 describes a platform structure having a platform and a floating structure, or buoyancy structure. In this case, the platform is connected to the seabed by means of tensioning cables.
DE 20 2007 009 474 U1 describes an offshore platform having a foundation pile, a transition piece, and a structure connection structure realized at the transition piece, wherein the transition piece is realized as a concrete construction.